1. Technical Field
The present invention relates to a fixing device employing an electromagnetic induction heating method, and to an image forming apparatus incorporating such a fixing device.
2. Related Art
Conventionally, an image forming apparatus such as a copier, a printer, and the like, includes a fixing device employing electromagnetic induction, which is both fast and energy-efficient.
For example, JP-2006-350054-A discloses a fixing device employing the electromagnetic induction heating method, which includes a support roller as a heat roller to generate heat, a fixing support roller as a fixing roller, a fixing belt stretching around the support roller and the fixing support roller, an induction heater disposed opposite the support roller via the fixing belt, and a pressure roller pressing against the fixing support roller via the fixing belt.
The induction heater is formed of a coil such as an excitation coil wound in the longitudinal direction and a core disposed opposite the coil. The fixing belt is configured to be heated at a portion opposite the induction heater. The thus-heated fixing belt heats to fix a toner image formed on a recording medium conveyed to a position opposite the fixing support roller and the pressure roller. More specifically, when a high frequency alternating current is supplied to the coil, an alternate magnetic field is formed around the coil, and an eddy current is generated near the surface of the support roller. When the eddy current is generated to the support roller as a heat roller, joule heat is generated by the electrical resistance of the support roller itself and the fixing belt wound around the support roller is heated.
The fixing device employing the electromagnetic induction heating method as described above has better thermal conversion efficiency and thus consumes less energy than a conventional halogen heater, and is capable of increasing a surface temperature of the fixing belt up to a prescribed level in a short time because a heat generator used in the electromagnetic induction fixing device is directly heated.
A coil used for the induction heating includes an excitation coil and a core to introduce alternate magnetic field generated by the excitation coil to the heat generator. The fixing device disclosed by JP-2008-032944-A includes a flux path by using cores 28, 29 from the excitation coil 25 to the fixing roller 20 including the heat generator.
As illustrated in FIGS. 2 and FIGS. 4A and 4B, the core 29 is divided into four parts A to D that are arranged around the circumference of the excitation coil with a minimum length, so that any leakage of the magnetic flux from the excitation coil is minimized and thermal efficiency is improved.
In addition, JP-2003-215957-A (or JP3452920) discloses a fixing device in which the excitation coil 5 is surrounded by the cores 32, 33, and 38 (see FIG. 16).
However, dividing the core as described above causes the magnetic flux to leak from joint portions between adjacent cores, thereby reducing heat generation efficiency. In addition, segmentation of the core increases the number of parts, resulting in a cost rise.
As an approach to the above disadvantage, provision of a gap between all arch-shaped cores and side cores is conceived to afford a unified contact status to reduce temperature fluctuation in the longitudinal direction due to dimensional variations of the opening. In this case, decrease in the heat generation efficiency cannot be prevented.
In addition, JP-2009-216751-A discloses a structure in which both side ends of the arch-shaped cores are bent in the direction to the heat generator. Specifically, FIG. 19 corresponds to FIG. 2 of the above patent literature. As illustrated in FIG. 19, both ends of the arch cores 54 are bent toward the heat roller 46, a heat generator.
In such a structure, heat rises at opposed surfaces of the heat generator, i.e., a front end of the bent portions of the arch-shaped cores, and therefore it is difficult to maintain a uniform temperature distribution along the axial direction of the roller or the coil longitudinal direction.
In addition, the excitation coil needs to be held in the arch-shaped core. However, the disclosed structure with both ends bent is unsuitable for mounting the arch-shaped cores from above the coil because bent portions at both ends interfere with the coil.